In both chronic HIV-1 and SIV infection, virus replication is concentrated in lymphoid tissues in follicular CD4+ T cells, which are 30 to 40 times more likely to be productively infected than extrafollicular CD4+ T cells. Reasons why lentiviruses are preferentially replicated by follicular CD4+ T cells and why virus-specific cytotoxic T lymphocytes (CTL) cannot suppress replication in these cells are not understood. Virus replication is not concentrated in B cell follicles in acute SIV infection, suggesting that te follicular concentration of virus may be related to adaptive immunity. T follicular helper cells (TFH) are a specialized subset of antigen-specific cells that migrate into B cell follicles where they interact with B cells and follicular dendritic cells (FDC) to promote B cell maturation and antibody production. We hypothesize that virus replication is concentrated within TFH in chronic HIV-1 and SIV infection due to 1) increased intrinsic susceptibility of TFH to productive lentiviru infection, 2) their location in germinal centers (GC) adjacent to antibody-virion complexes on FDC, and 3) numerical and functional deficiencies of follicular CTL. These hypotheses will be tested through a series of experiments both in vitro and in vivo using lymphoid tissues from humans and rhesus macaques. In Aim 1, we will establish whether human and macaque TFH are intrinsically more susceptible to productive lentivirus infection than other CD4+ T cells through in vitro experimental infections with GFP reporter viruses. In aim 2, we will determine the distribution and phenotype of productively infected T cells in vivo in both chronic and acute lentivirus infection by in situ tissue analyses as well as measuring viral RNA in sorted subsets of cells. In Aim 3 we will investigate the hypothesis that numerical and/or functional deficiencies in follicular CTL contribute to the propagation of HIV-1 within B cell follicles using in situ studies in vitro assays of follicular CTL function, and CD8 depletion of rhesus macaques in vivo. In the context of all aims, we will evaluate the impact of T follicular regulatory cells (TFR), on HIV-1 replication and CTL function. Collectively, these studies will provide a wealth of new information on the cells that foster HIV-1 replication in B cell follicles and factors in the follicular milieuthat may promote or impair lentivirus replication. A better understanding of the mechanisms that underlie permissiveness of follicular CD4+ T cells to lentiviruses is vital to development of a protective vaccine or a functional cure for HIV-1.